(1) Field of the Invention
The present invention relates to a method and a device for pouring a molten metal from a tundish into a mold in a continuous casting process, and more specifically to a method and a device for electromagnetically controlling the molten metal pouring rate in pouring a molten metal from a tundish into a mold in the continuous casting process.
(2) Description of the Prior Art
In the continuous casting of steel, the molten steel is supplied from a ladle into and stored temporarily in a tundish, and then the molten steel is poured into a mold from the tundish at a steady flow, to carry out continuous casting at a fixed casting rate. According to a conventional procedure of pouring the molten steel from the tundish into the mold, the molten steel pouring rate is regulated through the regulation of the level of the molten steel in the tundish when a tundish having a pouring nozzle of a small diameter is used, or through the regulation of the effective nozzle area by means of a stopper or a slide valve when a tundish having a pouring nozzle of a large nozzle area is used.
The former regulating method, however, is liable to cause the pouring nozzle of the tundish to clog, when the pouring temperature is low or in casting a steel with high aluminum content. Particularly, in casting billets of a small cross sectional area on a continuous casting machine, in which the molten steel needs to be poured at a low pouring rate, the molten steel needs to be maintained at a high temperature, which unavoidably entails the deterioration of the internal quality of the billet due to central segregation or cavities. Furthermore, this regulating method is incapable of being applied to manufacturing fine-grained steels with high aluminum content, because the pouring nozzle is clogged with alumina.
On the other hand, the latter regulating method employing a stopper for regulating the pouring rate is incapable of regulating the pouring rate satisfactorily, because only a slight change in the stroke of the stopper affects greatly the variation of the flow rate of the molten steel. While employment of a slide valve facilitates flow rate regulation, the slide valve is liable to result in air being sucked through the clearance between the sliding surfaces. The air thus sucked causes oxidation of the molten steel and increases the impurity content of the castings.
In consideration of the disadvantages of the conventional methods and devices, electromagnetic pumping devices for pouring a molten steel into a mold for continuous casting have been invented.
Molten steel pouring devices employing an electromagnetic driving mechanism of a linear motor type are described, for example, in "Technische Forschung Stahl", F. R. Block, 1980, and "Recherche Techniqueacier, 1980.
The electromagnetic driving mechanism of a linear motor type published in the former reference will be described hereinafter in connection with FIGS. 1 to 3. A tundish 1 is mounted on a portable table 2 so as to be tiltable on a support 3 (the tundish 1 is tilted by a hydraulic cylinder 4 to a position 1a indicated by alternate long and short dash lines after pouring). A ladle 5 is disposed over the tundish 1 to supply molten steel 6 to the tundish 1.
The tundish 1 has a refractory vessel 7 provided with a lid 8 and a supply trough 10 for pouring the molten steel 6 supplied from the ladle 5 into the tundish 1 into a mold 9.
The supply trough 10 extends diagonally upward from the refractory vessel 7 so that the highest position in the molten steel passage formed in the supply trough 10 is located above the level of the surface of the molten steel 6. The molten steel is driven by electromagnetic driving units 11 and 12 so as to flow over the highest position in the molten steel passage through an outlet 13 and into the mold 9. A reference numeral 14 designates a casting radius. The electromagnetic driving unit 11 is secured to the underside of the supply trough, while the other electromagnetic driving unit 12 is disposed movably on the topside of the supply trough 10.
FIG. 2(a) is a sectional view of the supply trough 10 and the lower electromagnetic driving unit 11, and FIG. 2(b) is a sectional view taken on line A--A in FIG. 2(a).
The molten steel passage 21 of a width 2a is formed in the supply trough 10. Coils 22 each wound around an iron core 23 are provided in the upper section of the electromagnetic driving unit 11. Although not shown in the drawings, plural sets each including a coil 22 and an iron core 23 are arranged longitudinally.
FIG. 3 is a sectional perspective view showing the lateral half of the electromagnetic driving unit 11 broken along the longitudinal centerline thereof and the molten steel 24 being transported. FIG. 3 further shows diagrammatically the respective values of a magnetic induction B and a current density i at a moment. A three-phase AC current is supplied to the coils 22. The three phases are arranged so that the pole pitch .tau. (half of the length of the period of variation of magnetic flux density) corresponds to three coils 22. Thus, the vectors B and i are produced, and thereby the molten steel is transported electromagnetically along the longitudinal direction of the supply trough 10 on the same principle as that of the well known linear motor.
The above-mentioned conventional electromagnetic molten steel supply trough incorporating a linear motor electromagnetic driving device has the following disadvantages. The inherent characteristics of the linear motor electromagnetic driving device require a long molten steel supply passage, which is liable to cause the temperature of the molten steel to drop while the molten steel is transported through the long supply trough. Furthermore, this conformation of the linear motor device has difficulty in producing a magnetic line of force which penetrates through the molten steel in the trough. Therefore, the inside diameter of the trough needs to be small for smooth transportation of the molten steel, and a large inside diameter increases the magnitude of the power required for molten steel transportation remarkably. Still further, the last portion of the residual molten steel needs to be discharged from the tundish by tilting the tundish, which requires a tilting mechanism.